1. Field of the Invention
This invention relates to a method of forming solutions and has particular reference to forming solutions of cellulose in tertiary amine N-oxide solvents.
2. Description of the Related Art
For many years shaped cellulose products such as fibres have been produced from cellulose by the formation of a cellulose compound which can be processed. Typically, the cellulose compound is cellulose xanthate.
More recently, proposals have been made to form shaped cellulose articles by forming true solutions of cellulose in a solvent and spinning the solvent to form a fibre or film.
A preferred form of solvent is a tertiary amine N-oxide, typically N-methylmorpholine N-oxide. Although cellulose can be dissolved in such cyclic compounds, it has proved difficult to dissolve it at rates which are commercially acceptable.
In U.S. Pat. No. 4,246,221 there is described a process for forming a true solution of cellulose in which there is prepared a premix of cellulose, a solvent such as a tertiary amine N-oxide and a non-solvent for cellulose such as water. A premix of the three components is made, and on heating the premix to evaporate the water, a true solution can be formed.
The U.S. patent referred to above describes forming the solution particularly by heating the mixture in an extruder. There is however description of alternative various types of equipment, in particular thin-film evaporators. Thin-film evaporators are well known as being suitable for the evaporation of moisture or liquid from a viscous solution. The premix of cellulose solvent and non-solvent is a viscous mixture.
An unusual aspect of the process of heating the premix and evaporating the water is that the result is to form a true solution of cellulose in the amine oxide. This solution is quite viscous--having a viscosity of typically 50,000 to 10,000,000 centipoise--and is more viscous than the premix which is the starting material. Thus, the resultant product from the thin-film evaporator has a higher viscosity than the starting material but is a true liquid whereas the premix is not. In normal operations the evaporation of a product from a material tends to result in the production of a solid as the material is processed.
For the production of a solution capable of being spun into fibres it is necessary to form a solution containing very little, if any, undissolved cellulose. It will be appreciated that the solution is formed into fibres by being spun or extruded through a jet plate containing many fine-diameter holes. Typically, the holes will have a diameter of 250 microns or less. Such holes quickly become blocked if the product passing through them is not a true solution.
Although the use of a thin-film evaporator is described in U.S. Pat. No. 4,246,221 and is illustrated in EP-A-0,356,419, it has been discovered that there are economic problems associated with the scaling-up of the process for forming a true solution from a laboratory-scale to a large commercial-scale system.
Laboratory-scale thin-film evaporators having four blades and a heated surface area of about 0.5 m.sup.2 have been found readily to produce a solution in accordance with the teachings of U.S. Pat. No. 4,246,221 and as illustrated in EP-A-0,356,419.
On attempting to carry out the same process utilising thin-film evaporators having a heated surface area of 1 m.sup.2 or above it has been found that a large increase in electrical (or rotor-induced) energy is required to obtain a complete solution of commercially viable quantities of the cellulose in the tertiary amine N-oxide. As the rate of production of solution is increased, there is a disproportionate increase in energy consumption.
A thin-film evaporator comprises an evacuable vertically oriented cylinder heated on the outside and containing a central rotatable shaft provided with paddle blades which distribute the material to be heated over the internal surface of the cylinder. As the central shaft is rotated the material passes down the evaporator, under the combined action of gravity and the paddles, to be subjected to heat and a vacuum within the thin-film evaporator. At the lower end of the cylinder the heated and processed material is removed by any suitable means. The normal arrangement for the paddles is to provide them in four vertical rows spaced at 90.degree. around the central shaft.
It has now been discovered that with larger thin-film evaporators (by which is meant thin-film evaporators having a heated surface area in excess of 1 m.sup.2) it is not possible to operate such a thin-film evaporator as described in the prior art in a method which will produce a satisfactory true solution of cellulose in tertiary amine N-oxide at an energy consumption rate in kilowatt hours of consumption per kg per m.sup.2 (kw/kg/m2) which is economic.